Blood is generally stored in flexible, collapsible containers. The soft, collapsible, plastic material of the containers permits the blood to be drained from the containers without bringing it into contact with vented air in the container.
Various plastic formulations exhibit good characteristics of compatibility with blood cells. At the present time, most blood bags are made of polyvinyl chloride formulations, for one reason because of good Radio Frequency (R.F.) sealing characteristics.
Other plastics are also promising candidates for collapsible blood bag materials, for example, various copolymers of polyolefins, such as those containing ethylene, propylene, and/or butylene units, and copolymers containing other units such as styrene and vinyl acetate.
Unfortunately, many vinylic polymer materials such as polyolefins are less susceptible to R.F. sealing. Thus, they are not as readily manufacturable by the peripheral heat sealing of plastic sheets together to define the container in the manner analogous to the present manufacturing process of vinyl blood bags, particularly when the most desirable thin-walled blood bags are being manufactured, in which the wall thickness is as low as 0.008 to 0.018 inch, for example. In this case, conventional assembly methods for blood bags have been found not to produde a container of adequate strength with, for example, polyolefin-based polymers having this low wall thickness.
Also, blood bags are usually of a generally oval cross section in which the major axis of the oval is substantially larger than the minor axis, e.g. at least 50 percent greater and preferably at least 100 percent greater.
Another alternative for manufacture of polyolefin-containing containers and the like is blow molding, in which a tubular parison is extruded, placed into a blow mold, and then inflated, to conform to the shape of the inner chamber of the mold. However, it has proven difficult to blow mold containers of extreme oval cross section without obtaining substantial differences in the wall thickness about the circumference of the cross section. This, in turn, interferes severely with the desired mode of collapsing of the container.
In accordance with this invention, novel oval containers are disclosed, having wall thicknesses about their cross section of improved uniformity, as well as other advantages described below. These containers can be made so that the container walls are sufficiently thin to be readily collapsible in the manner of conventional blood bags, although the use of the container is not restricted merely to blood bags, but they may be used for any of a large variety of desired uses for collapsible bags.